Techniques of tissue engineering employing biocompatible scaffolds provide viable alternatives to materials currently used in prosthetic and reconstructive surgery. These materials also hold promise in the formation of tissue or organ equivalents to replace diseased, defective, or injured tissues. In addition, biocompatible scaffolds can be used to form biodegradable materials that may be used for controlled release of therapeutic materials (e.g. genetic material, cells, hormones, drugs, or pro-drugs) into a predetermined area. However, most polymers used today to create these scaffolds, such as polylactic acid, polyorthoesters, and polyanhydrides, are difficult to control and result in, among other things, poor cell attachment and poor integration into the site where the tissue engineered material is utilized. Accordingly, focus has shifted to scaffolds formed from synthetic biomolecules, more particularly biomimetic scaffolds capable of in situ self-assembly.
The preparation of any synthetic material with structure on the nanoscale that mimics natural tissue is a challenging problem. One approach has been to prepare molecules that spontaneously assemble into fibrils similar in morphology to the proteins and proteoglycans that compose the natural extracellular matrix. In contrast to most synthetic biopolymers, the use of small, self-assembling molecules facilitates control of chemical and structural properties of these macromolecular assemblies.1-12 To that end, peptide amphiphiles have been shown to self-assemble under suitable conditions to form fibril-like micelles (referred to in the art as “nanofibers”), such nanofibers having particular utility as biocompatible scaffolds, more particularly in the area of tissue engineering.13-26 Previously disclosed peptide amphiphiles have been described as having peptide sequences identified through phage display methodology that are capable of non-covalently binding growth factors.27 [U.S. patent application Ser. No. 11/005,552, “Self-assembling peptide amphiphiles and related methods for growth factor delivery”, the entirety of which is included herein by reference] It is an object of the present invention to provide novel peptide amphiphiles that are superior to previously reported compounds, including modifications that elicit lower cytotoxicity and greater biocompatibility with chondrogenic cell types, more homogenous peptide blending and gelation under physiological conditions, while retaining the previously identified capability to bind the chondrogenic growth factor TGF-β1.27 It is a further object of the present invention to provide a method of using said improved TGF-β1 binding peptide amphiphiles to repair or regenerate defects in articular cartilage in vivo.49 This method represents a novel and potentially beneficial therapeutic treatment for patients with cartilage lesions (defects) on their joint surfaces resulting from acute injury or chronic degeneration.
Untreated articular cartilage lesions lead to pain, dysfunction, and accelerated osteoarthritis. Full thickness focal chondral lesions may progress to osteoarthritis, a disorder having an estimated economic impact approaching $65 billion in the U.S., when considering healthcare costs, loss of wages, and societal impact costs.28 Chondral lesions are found in a wide range of the population, including both the athletic cohort and older active patients. In a retrospective review of 31,516 knee arthroscopies, Curl et. al. found 53,569 cartilage lesions in 19,827 patients undergoing arthroscopy (2.7 lesions per knee, prevalence=63%).29 In a prospective study of 993 consecutive knee arthroscopies, Aroen et al. found an 11% incidence of focal chondral injuries.30 In knees that had any articular cartilage lesions, 20% were full thickness focal chondral lesions.
Focal articular cartilage lesions have limited regenerative potential. Several treatment modalities are currently in clinical use. The regenerative potential of untreated articular cartilage is limited to the formation of a fibrocartilage scar. Surgical strategies to regenerate hyaline or hyaline-like articular cartilage include abrasion arthroplasty; microfracture; implantation of cells, tissue, synthetics; and osteochondral plugs. Clinical and histological outcome studies of these techniques have demonstrated varying, and often confounding clinical results. Long term histological studies have demonstrated that the majority of the tissue regenerated in all of these techniques is fibrous with partial at best, often with no hyaline cartilage produced.31 
The limited self-healing capability of articular cartilage is largely due to the nature of the tissue. First, the avascularity of articular cartilage cannot support the formation of a fibrin clot. In vascularized tissues, this clot serves as a temporary matrix and a source of growth factors to stimulate natural healing, as seen in tissues such as in skin and bone. Second, the dense extracellular matrix (ECM) of articular cartilage restricts chondrocyte migration to the defect space. Third, chondrocytes have low mitotic activity, which results in insufficient cell proliferation and matrix synthesis for complete regeneration. With this limited natural healing capability, clinical intervention is necessary to prevent further articular cartilage degradation and early progression of degenerative osteoarthritis.
Microfracture is a common clinical procedure used for the repair of cartilage defects. The proposed benefits of microfracture are that it is a single-surgery procedure, relatively simple and cost-effective with low patient morbidity, and involves the patients' own mesenchymal stem cells (MSCs) as a cell source to facilitate cartilage regeneration. Its current clinical indications are in non-obese patients with a small, full-thickness contained focal defect. Another proposed benefit of microfracture is that it does not preclude the use of other cartilage restoration techniques at a later time. The regenerative process in microfracture involves a clot of multipotent MSCs that adhere to the subchondral bone. Histological assessment of microfracture in animal models and clinical testing have shown that over time most lesions substantiate into fibrous cartilage with predominant Type I collagen and limited Type II collagen. Additionally, clinically there is a significant decrease in functional outcome 18 months post-surgery, as well as in patients that are more than 40 years old.32 This suggests that there is deficient bioactivity, quantity, quality, and retention of chondrocyte cell phenotype within the defect. Additionally, there may be a paucity in the quantity of the extracellular matrix and availability of endogenous growth factors to induce chondrocyte differentiation.
Osteochondral transplantation (allograft and autograft) is another method whereby osteochondral graft plugs are used to recreate the chondral surface. While hyaline-like cartilage has been seen in a few reports, there is a high rate of failed integration of the graft with the surrounding cartilage.33 Donor site morbidity for autografts as well as other issues inherent with allograft (immunogenicity, strength of subchondral bone, chondrocyte viability, healing potential, bone integration, cost) have affected clinical and histological outcomes.34-36 
Other techniques for cartilage regeneration include cell implantation, with or without tissue engineered constructs. Clinical investigations have involved cell harvesting, expansion into monolayer and combination with a matrix or scaffold, followed by implantation. These techniques require a two-stage surgery, with a significantly higher cost and increased morbidity from the harvest site. Recently, clinical trials have retrospectively and prospectively evaluated these more advanced techniques with microfracture and shown equivocal results. In a long-term prospective randomized control trial, Knutsen et. al. found a 23% failure rate in each group at 5 years and more failures in the ACI group than microfracture at two years.37 33% of patients at 5 years had radiographic evidence of arthritis. In a recent systematic review of treatment of focal knee articular cartilage defects Magnussen et al. conclude that there is not one technique that is superior to another in clinical or histological evaluation.38 Clearly no clinical solution has been found that can provide a large percentage of patients with excellent outcomes at long term follow-up.
A proposed solution to the above challenges is a one-stage procedure that has limited morbidity, technical simplicity, and promotes retention of the chondrocyte phenotype in an articular defect by providing both a three-dimensional scaffold and appropriate chondrogenic growth factors with a high degree of bioactivity. The scaffold would ideally maximize chondrocyte and growth factor integration, viability, and function.